During the drafting of fiber bands in drafting devices, the clamping action of the roller pairs plays a decisive part for the transfer of the drafting forces onto the fiber band. The roller pairs of the drafting device therefore consist of a fluted steel cylinder, the so-called lower cylinder, and of a pressure roller, the so-called upper roller, that is pressed by load onto the steel cylinder. As a rule, this pressure roller comprises an elastic covering so that no clamping line is produced but rather a clamping surface formed by the deformation of the elastic covering, which surface brings about a significantly better fiber retention. A good clamping action is exerted on the fiber structure without damaging the fibers. Experience has shown that soft roller coverings therefore yield better drafting results since the softer the covering is, the greater is the clamping surface. However, soft roller coverings have the disadvantage that they wear down very rapidly and that grooves are produced in particular in the area of the fiber passage. This so-called “shrinking” is eliminated by buffing over the entire covering surface. This alters the geometry of the drafting device rollers and therewith also the covering properties, which for its part has a disadvantageous effect on the drafting conditions and thus on the yarn values. In addition, the regrinding of the roller coverings is a quite expensive measure.
The attempt has therefore already been made to counteract this disadvantage by means of a multi-layer roller covering. DE 1 815 739 U teaches a pressure roller whose elastic jacket is subdivided into at least two layers of which the outer layer is designed as an elastic casing consisting of a thin hose that can be drawn over the elastic jacket of the pressure cylinder. The designing of the outer layer as a hose makes it easy to draw a cover over the elastic jacket and also to easily remove it from the latter when this outer circumferential surface has become worn. The fixing of the elastic hose is ensured by the natural friction between rubber and rubber. This known design does make it possible to readily replace the elastic outer layer; however, it was not able to solve the problem of the rapid wear and of the shrinking.
DE 1 685 634 A1 teaches a covering for drafting device rollers of spinning machines which covering is composed of two superposed cylindrical layers of which the outer layer is harder and has a thinner wall than the inner layer. The two layers are adhered to one another. As a result thereof, very different materials can be combined with each other in order to avoid the formation of windings and electrostatic charging. However, it turned out that the problem of a good drafting ability and of wear were not satisfactorily solved. In addition, as a consequence of the adhesion the changing of the outer layer is expensive.